BB Code in signatures

[call_me_andrew]

Why do I keep getting the message "BB code url is not allowed" when I try changing my signature?

[Mark]

As we no longer allow links etc in new signatures due to them being excessively used by spammers.

[tfp]

Hi Mark, this may be a silly question but... I tried to change my signature and the same error message keeps appearing "BB code LEFT is not allowed"
What am I doing wrong? Apologies if this is a stupid question!
Cheers,
Will